锡田印支-燕山期复式花岗质岩浆-热液活动时限和物质来源

锡田钨锡多金属矿田位于湘赣两省交界,南岭成矿带的北部。锡田花岗岩体是一个复式岩体。本文利用LA-MC-ICP MS技术对8个样品的锆石进行了原位微区U-Pb定年和Lu-Hf同位素测试,结果显示其中有4个为印支期(226~225Ma)、另外4个为燕山期(166~158Ma)。印支期花岗岩锆石的εHf(t)值为-4.6~-10.8,两阶段模式年龄TDM2值为2011~1534Ma,根据是否呈正态分布可分为7组;燕山期花岗岩锆石的εHf(t)值为-4.3~-10,TDM2值为1896~1429Ma,可分为5组。相似的Hf同位素组成表明两期花岗岩具有同源性,均源自古元古代晚期至中元古代早期地壳物质的部分熔融。而εHf(t)值变化范围较大且分成很多组可能更主要受控于源区Hf同位素的不均一性。此外,本文利用40 Ar-39 Ar定年方法测得花岗岩顶部石英脉和云英岩中4个白(金)云母的年龄为150~148Ma,这期与成矿有关的热液作用应该与最晚一期岩浆活动有关。通过本次获得的定年数据和对已有的成岩成矿定年数据的总结,本文认为锡田岩体可以分为三个期次,印支期(226~225Ma),燕山期第一阶段(166~158Ma)和燕山期第二阶段(151~141Ma)。     

大兴安岭北段扎兰屯哈多河“花岗质糜棱片麻岩”年代学及地球化学特征研究

哈多河“花岗质糜棱片麻岩”为一套变质变形的花岗质岩石,其岩石组合为细粒黑云母二长花岗质糜棱片麻岩、细粒黑云母花岗质糜棱片麻岩和细粒石榴二云母花岗质糜棱片麻岩.SiO2含量在66.4％～74.52％之间,K2O+ Na2O介于7.36％～8.84％之间,Al2()3含量在13.43％～15.74％之间,A/CNK =0.974～1.092,属于高钾钙碱性、准铝质到过铝质岩石;具有较高的稀土总量(ΣREE=94.15×10-6～164.30×10-6),稀土元素配分模式表现为轻稀土元素(LREE)富集、重稀土元素(HREE)相对亏损的右倾型曲线,具有弱的负铕异常;富集Rb、Ba、U、K等大离子亲石元素,相对亏损Ta、Nb、P、Ti等元素.锆石U-Pb测年结果表明,哈多河“花岗质糜棱片麻岩”的岩浆侵入期次可分为两期,第一期在320Ma左右,代表性岩石为细粒黑云母花岗质糜棱片麻岩和细粒石榴二云母花岗质糜棱片麻岩;第二期在304Ma左右,代表性岩石为细粒黑云母二长花岗质糜棱片麻岩.综合岩相学、年代学及岩石地球化学特征,哈多河“花岗质糜棱片麻岩”原岩为高钾钙碱性Ⅰ型花岗岩,形成于晚石炭世兴安地块与松嫩地块的后碰撞构造环境下,之后在早二叠世末遭受“后碰撞作用”的影响,发生较强的韧脆性变质变形.     

黄沙坪矿区花岗岩类的锆石U-Pb年龄、Hf同位素组成及其地质意义

黄沙坪矿床是湖南最大的铅锌矿床。矿区内岩浆活动复杂,矿化类型齐全,成矿元素多样,是湘南地区斑岩-矽卡岩-热液脉型Cu多金属与W-Sn多金属复合成矿作用的典型代表。为厘清区内不同花岗质岩石的时间格架、源区特征及其与成矿的关系,本次研究对黄沙坪矿区的不同类型花岗岩进行了系统的锆石LA-ICP-MS U-Pb定年和Hf同位素组成研究,研究表明,英安斑岩、二长花岗斑岩和石英斑岩的侵位年龄分别为158.5±0.9Ma、155.2±0.4Ma、160.8±1.0Ma,石英斑岩内花岗质岩石包体可能形成于220.4±1.2Ma。另外,在二长花岗斑岩锆石中发现古元古代-新太古代的继承锆石核。不同花岗质岩石中,中晚侏罗世锆石的εHf(t)值为-7.6～-3.2,Hf同位素两阶段模式年龄为1.7～1.4Ga,表明该区花岗质岩浆主要源自中元古代的古老基底物质部分熔融。继承锆石中接近0的εHf(t)负值(-1.5～-0.07)和εHf(t)正值(0.5～6.5)暗示形成这些古老继承锆石的初始物质中有幔源物质的加入。黄沙坪矿区不同类型花岗质岩石的锆石年龄和Hf同位素特征表明,英安斑岩、二长花岗斑岩、石英斑岩可能是同源同时期岩浆演化的产物,与矿床成矿年龄(154～159Ma)接近,反映其成岩成矿具有密切的时空联系。花岗岩中存在多组古老的残留锆石,暗示了黄沙坪地区自新太古代以来经历了复杂的岩浆作用,这可能是区内Cu-Pb-Zn-W-Mo-Fe多金属复合型矿床形成的关键要素。     

宜章县长城岭浅成花岗质岩浆隐爆机制及找矿意义探讨

在长城岭铅锌多金属矿区,常见沿花岗斑岩脉与北东向断裂交汇处发育与成矿关系密切的角砾岩。这类角砾岩的空间就位、物质组成、岩石组构及蚀变特征等表明其为隐爆成因,可划分为爆破角砾岩、震碎角砾岩和混杂角砾岩。按照隐爆机制的不同,其又可分为岩浆隐爆角砾岩和热液隐爆角砾岩。区内花岗斑岩的地质地球化学特征表明,燕山早期晚阶段侵入的富含挥发分、流体-溶体相互作用强烈的浅成花岗斑岩具备有利的隐爆条件和明显的隐爆特征。隐爆作用不仅为铅锌多金属矿化富集提供物源及流体源,还为成矿流体汇聚及卸载沉淀提供了重要场所。长城岭地区与铅锌多金属成矿关系密切的隐爆角砾岩的发现对其矿区深部、湘南钨锡多金属矿化集中区乃至南岭中段有色金属找矿研究具有重要意义。     

华北板块北缘古大洋闭合时间的限定 ——来自四子王旗西后壕子同碰撞花岗岩的证据

位于华北板块北缘四子王旗北部的西后壕子花岗岩体的Cameca IMS-1280-SIMS锆石U-Pb年龄为266Ma±2Ma。该岩体主要由二长花岗岩组成,以出现大量颗粒较大的白云母为特征,具有较高的SiO2含量（75.21%~76.69%）,A/CNK值多大于1.1,CIPW标准矿物中刚玉的含量也都高于1%。在球粒陨石标准化稀土元素配分图解上,Eu具有强烈的负异常（δEu=0.03~0.11）,REE呈现出明显的M型四分组效应,部分微量元素也表现出non-CHARAC的性质。在原始地幔标准化微量元素配分图上,元素Ba、Nb、Sr和Ti相对亏损,Cs、Rb、Th和Pb相对富集。在构造环境判别图解中,所有样品均落入同碰撞花岗岩区域。结合区域地质背景及以上特征,西后壕子岩体应为强过铝质的高分异S型花岗岩,形成于二叠纪晚期的陆-陆碰撞环境中。270~260Ma期间,四子王旗地区发育EW向的碰撞花岗岩带,限定华北板块与南蒙古微大陆沿索伦缝合带的闭合时间为二叠纪晚期。     

大别山地区片麻岩套的建立与成岩时代讨论

本文根据大别山地区的详细地质填图和研究资料，对“大别杂岩”、“大别片麻岩套”的建立及其产出背景，片麻岩套的主要特征（分布产状、岩石类型、岩石地球化学特征等）作了较系统阐明，最后就其形成机制（岩浆起源、形成深度、温压条件等）、成岩时代（晋宁晚期到华力西期）作了分析，为正确认识大别山变质造山带的形成演化和高压一超高压岩石形成的“高压釜”说提供了重要地质依据。     

Precambrian geotectonic units of the Río de La Plata craton

The main Precambrian tectonic units of Uruguay include the Piedra Alta tectonostratigraphic terrane (PATT) and Nico Pérez tectonostratigraphic terrane (NPTT), separated by the Sarandí del Yí high-strain zone. Both terranes are well exposed in the Río de La Plata craton (RPC). Although these tectonic units are geographically small, they record a wide span of geologic time. Therefore improved geological knowledge of this area provides a fuller understanding of the evolution of the core of South America. The PATT is constituted by low- to medium-grade metamorphic belts (ca. 2.1 Ga); its petrotectonic associations such as metavolcanic units, conglomerates, banded iron formations, and turbiditic deposits suggest a back-arc or a trench-basin setting. Also in the PATT, a late to post-orogenic, arc-related layered mafic complex (2.3–1.9 Ga), followed by A-type granites (2.08 Ga), and finally a taphrogenic mafic dike swarm (1.78 Ga) occur. The less thoroughly studied NPTT consists of Palaeoproterozoic high-grade metamorphic sequences (ca. 2.2 Ga), mylonites and postorogenic and rapakivi granites (1.75 Ga). The Brasiliano-Pan African orogeny affected this terrane. Neoproterozoic cover occurs in both tectonostratigraphic terranes, but is more developed in the NPTT. Over the past 15 years, new isotopic studies have improved our recognition of different tectonic events and associated processes, such as reactivation of shear zones and fluids circulation. Transamazonian and Statherian tectonic events were recognized in the RPC. Based on magmatism, deformation, basin development and metamorphism, we propose a scheme for the Precambrian tectonic evolution of Uruguay, which is summarized in the first Palaeoproterozoic tectonic map of the Río de La Plata craton.     

Late Palaeoproterozoic post-collisional magmatism in the North China Craton: geochemistry,zircon U–Pb geochronology,and Hf isotope of the pyroxenite–gabbro–diorite suite from Xinghe,Inner Mongolia

The North China Craton (NCC) witnessed a prolonged subduction–accretion history from the early to late Palaeoproterozoic, culminating with final collision at ca. 1.85 Ga and assembling the continental blocks into the cratonic framework. Subsequently, widespread post-collisional magmatism occurred, particularly along the Trans-North China Orogen (TNCO) that sutures the Eastern and Western blocks of the NCC. Here we present petrological, geochemical, and zircon U–Pb geochronological and Lu–Hf data from a pyroxenite (websterite)–gabbro–diorite suite at Xinghe in Inner Mongolia along the northern segment of the TNCO. The internal structures and high Th/U values of the zircons from the gabbro–diorite suite suggest magmatic crystallization. LA-ICP-MS U–Pb age data on three gabbros and one diorite from the suite yield emplacement ages of 1786.1 ± 4.8, 1783 ± 15 ,1754 ± 16 and 1767 ± 13 Ma, respectively. The ε_Hf(t) shows mostly positive values (up to 5.8), with the lowest value at –4.2, suggesting that the magma was derived from dominantly juvenile sources. The generally low SiO₂ and high MgO values, and other trace element features of the Xinghe suite are consistent with fractionation from a mantle-derived magma with a broadly E-MORB affinity, with no significant crustal contamination. Recent studies clearly establish that the major magmatic pulse associated with rifting of the NCC within the Columbia supercontinent occurred in the late Mesoproterozoic at ca. 1.3–1.2 Ga associated with mantle plume activity. This, together with the lack of robust geochemical imprints of rift-related magmatism in the Xinghe suite, prompts us to suggest a tectonic model that envisages magma genesis associated with post-collisional extension during slab break-off, following the westward subduction of the Eastern Block and its collision with the Western Block. The resulting asthenospheric upwelling and heat input might have triggered the magma generation from a heterogeneous, subduction-modified sub-lithospheric mantle source for the Xinghe rocks, as well as for similar late Palaeoproterozoic suites in the TNCO.     

Zircon U–Pb geochronology and Hf isotopic constraints on the petrogenesis of Early Triassic granites in the Wulonggou area of the Eastern Kunlun Orogen,Northwest China

Widespread granitic intrusions in the northeast part of the Wulonggou area were previously thought to be emplaced into the Palaeoproterozoic Jinshuikou Group during the Neoproterozoic. This contribution presents detailed LA-ICP-MS zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf isotope systematic on the Wulonggou Granodiorite and Xiaoyakou Granite from the Wulonggou area. Three granodiorite samples yielded U–Pb zircon ages of 247 ± 2, 248 ± 1, and 249 ± 1 Ma, and one granite sample yielded U–Pb zircon age of 246 ± 3 Ma. The granodiorite samples are metaluminous with an alumina saturation index of 0.90–0.96, as well as intermediate- to high-alkali contents of 5.49–6.14 wt.%, and low Zr+Nb+Ce+Y contents, and low Fe₂O_3T/MgO ratios, which suggest an I-type classical island arc magmatic source. The granite samples are peraluminous with an alumina saturation index of 1.02–1.03, Sr content of 305.00–374.00 ppm, Sr/Y ratios of between 17.68 and 28.77, (La/Yb)_N values of 16.98–25.07, low HREEs (Yb = 1.10–2.00 ppm), and low Y (13.00–21.10 ppm), which suggest adakite-like rocks. All granodiorite samples have zircons ε_Hf(t) values ranging from ?2.9 to +3.9, and granite samples have zircon ε_Hf(t) values ranging from ?7.8 to +3.2. These Hf isotopic data suggest that the Early Triassic granites were derived from the partial melting of a mafic Mesoproterozoic lower crust, although the degree of ancient crustal assimilation may be higher for the Xiaoyakou Granite. It is suggested here that the ca. 246–248 Ma magma was generated during the northward subduction of the Palaeo-Tethys oceanic plate.     

Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan,North Qaidam belt,NW China

abstract

An integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust.